1. Field of the Invention
The present invention relates to a method for generating print data for a digital color image having pixels, the method comprising the steps of determining for each pixel in the digital color image a value of a characteristic feature, and selecting an available conversion method for processing the pixels of the digital color image into print data, based on the determined characteristic values of the pixels. The present invention further relates to a color printing system comprising a controller for scheduling and interpreting print jobs and a printing process for marking support material, the controller comprising modules for generating print data for a digital color image having pixels.
2. Background of the Invention
In the state of the art, color printer systems are applied for producing high quality reproductions of images on a receiving material. These images exist in the form of a digital representation and originate from a computer application, from a camera, from a scanner, or any other source. At the time of reproduction, the images are prepared to be part of a print job that is sent to the printer system. The printer system comprises a printing engine and a controller for scheduling and interpreting print jobs. In the controller, an image of a print job is converted to print data that is sent to the printing engine for marking the receiving material. A printing engine comprises, e.g. an electrophotographic process, wherein an illumination system is used to imagewise discharge a charged photoconducting material. Thereafter the photoconducting material is developed by dry colored powder, which is transferred to the receiving material. Another printing engine comprises an inkjet process in which inkjet heads release imagewise small ink drops of various colors from nozzles on a side of the printhead where the receiving material is brought close to. In support of each printing process, the image is converted in a number of processing steps to print data that is suitable for the printing process. This is called a conversion method and comprises, e.g. background reduction, to reduce the amount of ink or toner that is rendered in the background of the information in the image, contrast enhancement, to accommodate the contrast in the image to the contrast that is available with the toner or ink on the receiving material, filtering for sharpness improvement and prevention of moiré, halftoning for deciding where the dots that form the reproduction of the image on the receiving material are placed and print strategy masking, which is used to determine which pixel positions are printed at which printing process pass. These processing steps are well known in various forms.
A color printer system as indicated is capable of processing and printing many kinds of color images. Some of the images have only lines and text on a white background; other images comprise photographic images or maps. Maps comprise images that have text on a colored background. If the image does not have pixels, the digital representation of the image is processed to a pixel-based digital representation. Each pixel is characterized by a combination of numbers for a number of channels. In one case, each pixel is characterized by three numbers, one for the amount of red in the R-channel, one for the amount of green (G-channel) and one for the amount of blue (B-channel). In another case, each pixel is characterized by four numbers for cyan, magenta, yellow, and black. In each case, the combination of numbers for the channels is called the value of the pixel. It has been found that each type of image has an optimal conversion method for generating print data. Therefore, it is advantageous to select the appropriate conversion method for an image of a defined type. Usual image types include 2D CAD, which comprise line drawings and text, 3D CAD/maps, which comprise business graphics, model drawings and geographic maps, and photographic images, which comprise images with real-life scenes.
In the current state of the art, the user of the print system is asked to select a conversion method for a print job. If the print job comprises only one image type, the most appropriate one may be selected, assuming the user is aware of the possibilities of the printer system. In mixed print jobs, which comprise various image types, the user has the choice to either use a print conversion method that is suboptimal for some of the images in the job, or to split the print job into several print jobs, each involving a single type of image.
In U.S. Patent Application Publication No. 2005/0163385, a method is described to classify images using a concentration ratio, which is compared to one or more classification thresholds. This concentration ratio is determined from histogram data of the lightness of the pixels in the image. The classification can be used to select appropriate settings, i.e. the appropriate conversion method, for printing the image. However, the latitude of this method is rather limited, since all characteristic features of the image are expressed in one number.